1. Field of the Invention
The present invention generally concerns optical switches; particularly optomechanical switches; and still more particularly micromachined, or Micro Electro Mechanical Systems (MEMS), optomechanical switches.
The present invention particularly concerns micromachined, or Micro Electro Mechanical Systems (MEMS), optomechanical switches having a MEMS torsion platexe2x80x94which serves to mount a micromirrorxe2x80x94that is (i) electrostatically or electromagnetically moved in angular position, and/or (ii) hinged for angularly pivoting movement by surface micromachined hinges, and/or (iii) mechanically biased in angular privoting movement by torsion springs that also serve to conduct electricity.
2. Description of the Prior Art
2.1 General Prior Art Optical Switching, and Micromachined Optical Switches
Optical switching plays a very important role in telecommunication networks, optical instrumentation, and optical signal processing systems. In telecommunication networks, fiber optic switches are used for network restoration, reconfiguration, and dynamic bandwidth allocation.
There are many different types of optical switches. In terms of the switching mechanism, the switches can be divided into two general categories. A first type, called xe2x80x9coptomechanical switchesxe2x80x9d, involves physical motion of some optical elements. The second type, which will be referred to here as xe2x80x9celectro-optic switchesxe2x80x9d, employs a change of refractive index to perform optical switching. This refractive index change can be induced by electro-optic, thermal-optic, acousto-optic, or free-carrier effects. The electro-optic-type switch generally needs to be implemented in coupled optical waveguides.
Optomechanical switches offer many advantages over electro-optic switches. They have lower insertion loss, lower crosstalk, and higher isolation between ON and OFF state. They are bidirectional and independent of optical wavelength, polarization, and data modulation format. The crosstalk of electro-optic waveguide switches is limited to a range above xe2x88x9230 dB, and is often in the range of xe2x88x9210 to xe2x88x9215 dB, while optomechanical switches can routinely achieve crosstalk  less than xe2x88x9250 dB.
An optomechanical switch can be implemented either in free-space or in waveguides (or in fibers). The free-space approach is more scalable, and offers lower coupling loss to optical fibers. Currently, macro-scale optomechanical switches employing external actuators are available commercially. For example, conventional optomechanical switches are available from JDS, DiCon, AMP, HP, etc. Most, of these switches are, however, bulky and require extensive manual assembly. Their speed is also slow, the switching time ranging from 10 milliseconds to several hundred milliseconds. Worse, the switching time often depends on the switching path, i.e., how far is the next output port from the current output port. This is very undesirable for system design. Response times below 1 millisecond are desirable for network applications.
Meanwhile, micromachining technology, also known as Micro Electro Mechanical Systems (MEMS), offers many advantages for building optomechanical switches. MEMS technology is a batch processing technique for fabricating movable microstructures and microactuators. See, for example, K. E. Petersen, xe2x80x9cSilicon as a Mechanical Materialxe2x80x9d, Proc. IEEE 70 (1982) 420-457; and M. F. Dautartas, A. M. Benzoni, Y. C. Chen, G. E. Blonder, B. H. Johnson, C. R. Paola, E. Rice, and Y. H. Wong, xe2x80x9cA Silicon-Based Moving Mirror Optical Switchxe2x80x9d J. Lightwave Technol. 8 (1992) 1078-1085.
Micromachining technology, or MEMS, can significantly reduce the size, weight, and cost of optomechanical switches. The switching time can also be reduced because of the higher resonant frequency of the smaller optomechanical switches. Furthermore, the MEMS optomechanical switch is more rugged than macro-scale switches of equivalent design because the inertial forces are much smaller in the micro-scale switches. MEMS technology has previously been employed to realize various types of optomechanical switches.
2.1 Specific Prior Art Micromachined Optical Switchesxe2x80x94Free-Space Optomechanical Switches
There has been some demonstration of MEMS fiber optic switches. Both bulk-micromachining and surface-micromachining techniques have been employed. However, none of the reported switches fully satisfy all the requirements for large scale network applications. The bulk micromachined 2xc3x972 fiber optic switch was reported by ATandT Bell Labs in 1992. See M. F. Dautartas, A. M. Benzoni, Y. C. Chen, G. E. Blonder, B. H. Johnson, C. R. Paola, E. Rice, and Y. H. Wong, xe2x80x9cA Silicon-Based Moving Mirror Optical Switch,xe2x80x9d in J. Lightwave Technol, 8 (1992) 1078-1085. The Bell Labs switch employs two separate silicon wafers (vertical micromirrors in the top  less than 110 greater than  silicon wafer, and V-grooves grooves in bottom  less than 100 greater than  silicon substrate). The two wafers are joined together manually and external actuators are employed. An insertion loss of 0.7 dB and a switching time less than 10 ms have been obtained. This switch, however, still requires substantial manual assembly and the cost is very high. It does not appear to be scalable to large arrays.
Toshiyoshi and Fujita of the University of Tokyo reported a 2xc3x972 matrix switch using bulk micromachined torsion mirrors. See Toshiyoshi, H.; Fujita, H. xe2x80x9cElectrostatic micro torsion mirrors for an optical switch matrix,xe2x80x9d J. Microelectromechanical Systems, vol. 5 , p. 231-7, 1996. Torsion mirrors are suspended by thin polysilicon beams over through holes etched on silicon substrate. The mirror substrate is then bonded to another silicon substrate, on which bias electrodes and mechanical stops are patterned and etched. When the mirror is attracted downward by the electrostatic force, light is reflected to the orthogonal output fibers. Large switching contrast ( greater than 60 dB), small crosstalk ( less than 60 dB), and a fairly high insertion loss of 7.6 dB were reported. One limitation of this approach is that the mirror angle in the ON state is determined by the mechanical stop on another wafer and cannot be accurately controlled or reproduced. This results in the high insertion loss reported in their paper.
Marxer et al., from University of Neuchatel reported a bulk micromachined 2xc3x972 fiber optic switches using deep reactive ion etching (DRIE) process on silicon-on-insulator (SOI) wafer. See C. Marxer, M.-A. Gretillat, N. F. de Rooij, R. Battig, 0. Anthamatten, B. Valk, and P. Vogel, xe2x80x9cVertical Mirrors Fabricated by Reactive Ion Etching for Fiber Optical Switching Applications,xe2x80x9d in Proc. 10th Workshop on Micro Electro Mechanical Systems (MEMS), pp. 49-54, 1997. The 75-xcexcm-thick silicon layer above SiO2 is etched through by DRIE. A 2.3-xcexcm-thick vertical mirror as well as the electrostatic microactuator are created by the same etching step. Switching time below 0.2 ms, coupling loss of 2.5 dB and 4 dB, and switching voltage of 28 V have been achieved. A similar process has recently been reported by the Michigan group. See W. H. Juan and S. W. Pang, xe2x80x9cBatch-Micromachined, High Aspect Ratio Si Mirror Arrays for Optical Switching Applications,xe2x80x9d in Proc. International Conf. Solid-State Sensors and Actuators (TRANSDUCERS 97), Paper 1A4. 09P, 1997.
There are two limitations of this approach: (1) the mirror angle and quality are determined by the etching process. Mirror angle  less than 1xc2x0 is difficult to achieve. The rough etched surface also introduces scattering losses. It is also difficult to polish the sidewalls or coat metals on the mirrors. (2) The displacement of the mirror is small because the comb drive actuator has limited displacement. In Marxer""s paper, tapered fibers were placed very closed to the mirror ( less than 20 xcexcm), which reduces the required mirror movement. However, such configuration is not scalable to switches with dimension larger than 2xc3x972 arrays. Michigan""s group did not report the optical characterization data of their switch.
Miller and Tai of Caltech reported an electromagnetically actuated 2xc3x972 fiber optic switches. See R. A. Miller, Y. C. Tai, G. Xu, J. Bartha, and F. Lin, xe2x80x9cAn Electromagnetic MEMS 2xc3x972 Fiber Optic Bypass Switch,xe2x80x9d in Proc. International Conf. on Solid-State Sensors and Actuators (TRANSDUCERS 97), pp. 89-92, 1997. The 5-xcexcm-thick micromirror is fabricated by anisotropic (TMAH) etching on  less than 011 greater than  silicon substrate. The mirror is then glued a suspended silicon membrane. The silicon membrane is bulk micromachined, and connected to the silicon substrate by cantilevers or torsion beams. It is actuated electromagnetically through the interaction of an integrated copper coil and an external magnet. A switching time of 10 ms, an insertion loss of 3 dB, and switching current of 20-30 mA have been achieved. There are several limitations for this approach: first, the electromagnetic coil has large fringe field and, therefore, high crosstalk among adjacent switching elements. It is difficult to make dense switch arrays. Second, the power consumption for each switch is very high. Heat sinking will be a major issue for large arrays. Third, constant power consumption is required. Forth, the mirror integration process is not a manufacturable process.
In 1995, UCLA reported a surface-micromachined 2xc3x972 fiber optic switch. See S. S. Lee, L. Y. Lin, and M. C. Wu, xe2x80x9cSurface-Micromachined Free-Space Fiber Optic Switches,xe2x80x9d Electronics Letters, Vol. 31, No. 17, pp. 1481-1482, August 1995. Surface micromachining offers greater flexibility since it can monolithically integrate various types of three-dimensional micro-optical elements with microactuators. See M. C. Wu, L. Y. Lin, S. S. Lee, and K. S. J. Pister, xe2x80x9cMicromachined Free-Space Integrated Micro-Optics,xe2x80x9d in Sensors and Actuators: A. Physical, Vol. 50, pp. 127-134, 1995.
The UCLA switch is made of a surface-micromachined moveable micromirror. The vertical micromirror was realized by the microhinge technology. See K. S. J. Pister, M. W. Judy, S. R. Burgett, and R. S. Fearing, xe2x80x9cMicrofabricated Hinges, Sensors and Actuators: A. Physical, Vol. 33, pp. 249-255, 1992. This vertical micromirror was integrated on a surface-micromachined translation stage. Because stepper motor (scratch drive actuator) is employed, and large travel distance is needed, the switching time is inherently long (15 msec). See S. S. Lee, E. Motamedi, and M. C. Wu, xe2x80x9cSurface-Micromachined Free Space Fiber Optic Switches with Integrated Microactuators for Optical Fiber Communication Systems,xe2x80x9d in Proc. 1997 International Conferences on Solid-State Sensors and Actuators Transducers 97, Paper 1A4.07P, 1997. This switch also suffers from very poor reliability due to friction and tear/wear (similar to that of the original micromotors). See L. S. Fan, Y. C. Tai, and R. S. Muller, xe2x80x9cIntegrated Movable Micromechanical Structures for Sensors and Actuators,xe2x80x9d in IEEE Trans. Electron. Device, Vol. 35, pp. 724-730, 19E8.
The torsion mirrors such as the Digital Micromirror Devices (DMD) (sometimes xe2x80x9cDeformable Mirror Device) and their variations developed by Texas Instruments can also be employed as an optical switch. See, as an early patent on a DMD, U.S. Pat. No. 4,441,791 to Hornbeck for a DEFORMABLE MIRROR LIGHT MODULATOR and assigned to Texas Instruments Incorporated (Dallas, Tex.). The Hornbeck DMD concerns a light modulator where a light-reflective metallized membrane defining a deformable mirror is disposed over a semiconductor substrate in which a matrix array of field effect address transistors are formed.
The modern form of the DMD is basically a torsion mirror with xc2x110xc2x0 tilting angles. Two types of optical switches have been realized by DMD-like devices: the first type employs the DMD as tilting mirrors. Light from input fiber is directed to different output fibers by the DMD. This switch is limited to 1xc3x972 arrays and is very difficult to scale up to larger switches. Packaging is also very difficult since the optical fibers are not in the same plane as the switches. The second type uses the DMD as an ON-OFF switch. For this application, the asymmetric DMD, i.e., the torsion plate with a very long arm, is often employed. To construct a Nxc3x97N switch, each fiber is split into N such ON-OFF switches, and the output to the same fiber is then combined from N switches again. This switch is fundamentally lossy. The fundamental splitting loss is 2 log N, or 18 dB for an 8xc3x978 switch, which is unacceptable for system application without employing optical amplifiers.
Another ON/OFF switch, called in-line fiber gate, was demonstrated by the NTT group. See E. Hashimoto, Y. Uenishi, K. Honma, and S. Nagaoka, xe2x80x9cMicro-Optical Gate for Fiber Optic Communications,xe2x80x9d in Proc. International Conf. on Solid-State Sensors and Actuators (TRANSDUCERS 97), pp. 331-334, 1997. Hashimoto, et al. employ the electrostatic torsion beam actuator similar to the Digital Micromirror Device (DMD) of Texas Instruments, Inc, (Dallas, Tex.). Instead of using the torsion plate as mirror, another vertical metal mirror is integrated on the torsion plate by electroplating. The height of the micromirror is about 20 xcexcm, and the precise angle of the micromirror is hard to control (an accuracy of 0.5xc2x0 requires an aspect ratio of 115, while most thick photoresist mold""s aspect ratio is less than 20). The displacement of the torsion plate is also very small due to the thin sacrificial layer used. The displacement requirement is relaxed by employing lensed optical fibers, which reduce the optical beam diameter to 2 xcexcm. However, they are not suitable for large switches. The ON/OFF switch is also undesirable for constructing Nxc3x97M switches.
2.3 Specific Prior Art Micromachined Optical Switchesxe2x80x94Waveguide/Fiber-Based Optomechanical Switches
Ollier and Mottier reported a 1xc3x972 waveguide optical switch. See Ollier, E., Mottier, P., xe2x80x9cIntegrated electrostatic micro-switch for optical fibre networks driven by low voltage,xe2x80x9d Electronics Letters, vol. 32, p. 2007-9, 1996. The device consists of a movable waveguide and two fixed waveguides. The movable waveguide is basically a silica cantilever beam realized by undercutting the silicon below the waveguide. The motion is constrained to the in-plane direction by two flexure beams. The movable waveguide can be pulled towards either fixed waveguides by electrostatic gap-closing actuators. A fiber-to-fiber insertion loss of 3-4 dB, a switching time of 0.8 ms, and a driving voltage of xc2x128 V has been obtained. Another type of fiber optic switch involves physical movement of the fiber itself instead of waveguide Field, et al. of HP reported a 1xc3x972 fiber optic switch which utilize a electroplated (LIGA) thermal actuator to move the input fiber between two output fibers. See L. A. Field, D. L. Burriesci, P. R. Robrish, and R. C. Ruby, xe2x80x9cMicromachined 1*2 optical-fiber switch,xe2x80x9d Sensors and Actuators A (Physical), Vol. A53, pp. 311-16, 1996. A coupling loss of xe2x88x925.8 dB and isolation of 66.5 dB have been achieved. Another moving-fiber optical switch is reported by Gonzales and Collins. See C. Gonzalez and S. D. Collins, xe2x80x9cMagnetically actuated fiber-optic switch with micromachined positioning stages,xe2x80x9d Optics Letters, Vol. 22, pp. 709-11, 1997. The 1xc3x974 switch consists of one movable fiber and four fixed fibers. The movable fiber is mounted on a bulk-micromachined and manually assembled translation stage, which is actuated by an electromagnetic actuator. A thin film of paramagnetic material (permalloy) is deposited on the input translation stage and an external magnetic field is applied to either side of the sliding stage to align the input fiber to the appropriate output fiber. A switching time of 5 msec, an insertion of 1 dB, and a crosstalk of xe2x88x9260 dB have been achieved for multimode fibers. One of the major constraints of the waveguide-based fiber optic switches is that they are mainly constrained to 1xc3x97N switches, and it is very difficult to scale up to Nxc3x97M switches. The waveguide-type switches also have high fiber-to-fiber insertion loss.
2.4 Background Regarding the Alignment(s) of Free Space Optomechanical Switches
The present invention will be seen to facilitate, and improve, precise optical alignments in free space, and through micromechanical optomechanical switches. A patent discussing the requirements of optical alignment in optical switching is U.S. Pat. No. 5,155,778 to Magel, et. al. for an OPTICAL SWITCH USING SPATIAL LIGHT MODULATORS (assigned to Texas Instruments Incorporated, Dallas, Tex.). This patent concerns a structure for optical interconnection. In one embodiment, the structure consists of optical fibers connected to an array of microlenses, either through integrated waveguides or not, that direct light onto a mirror formed in a substrate, which mirror reflects light to a spatial light modulator. The spatial light modulator in turn reflects the light back to another mirror, which reflects the light through another microlens array, through integrated waveguides or not, and out another optical fiber. The structure is manufactured by forming the mirrors out of the substrate, forming waveguides if desired, forming troughs for the fibers and the microlenses, attaching external pieces such as the fibers, the lenses, and the spatial light modulator package, and packaging the device to maintain alignment.
2.5 Background Regarding the Tilt Angle of Free Space Optomechanical Switches
The performance of DMD-devices is also of interest. The devices of the present invention do not directly have a xe2x80x9ctilt anglexe2x80x9d though which light is reflected, but will be seen to have the ability to selective (i) transmit, or (ii) reflect, incipient light beam, for an effective xe2x80x9ctilt, anglexe2x80x9d of 180xc2x0. It is, however, illustrative to understand how important a large (180xc2x0 is maximum) and regularly repeatable xe2x80x9ctilt anglexe2x80x9d is in free-space optical switching. Such an understanding can be gained from U.S. Pat. No. 5,548,443 to Huang for a LIGHT SEPARATOR FOR TESTING DMD PERFORMANCE assigned to Texas Instruments Incorporated (Dallas, Tex.). The Huang light separator tests the tilt angles of mirror elements of a digital micro-mirror device. The light separator is comprised of two triangular prisms. A bottom prism receives light from all mirror elements. It transmits light from all mirror elements having a tilt angle over a specified angle from a different face than light from mirror elements having a tilt angle less than the specified angle. A top prism receives light from one face of the bottom prism. It further divides the light, so that light from all mirror elements having a tilt angle within a specified range is transmitted from one face and light from other mirror elements is transmitted from another face.
2.6 Background Regarding the Sticking, Adherence, or xe2x80x9cStictionxe2x80x9d of the Moving Elements of Optomechnical Switches
The present invention will shortly be seen to offer improvement regarding the sticking, or adherence, or xe2x80x9cstictionxe2x80x9d of the moving element(s) of an optomechanical switches.
A previous attempt to improve operability in this area is shown in U.S. Pat. No. 5,579,151 to Cho for a SPATIAL LIGHT MODULATOR assigned to Texas Instruments Incorporated (Dallas, Tex.). The Cho Spatial Light Modulator, or SLM, has a reflector that is electrostatically deflectable out of a normal position, where a supporting beam is unstressed, into a deflected position, where a portion of the mirror contacts a portion of a landing electrode which is at the same electrical potential as is the reflector. An inorganic layer or solid lubricant is deposited on the contacting portions. After the modulator is operated for a period of time, the tendency of the reflector to stick or adhere to the landing electrode is diminished or eliminated by the layer so that the reflector is returned to its normal position without any reset signal or with a reset signal having a reasonably low value. Preferred materials for the layer are SiC, AlN or SiO(2).
Yet another previous effort to deal with this problem is shown in U.S. Pat. No. 5,665,997 to Weaver, et. al. for a GRATED LANDING AREA TO ELIMINATE STICKING OF MICROMECHANICAL DEVICES assigned to Texas Instruments Incorporated (Dallas, Tex.). The Weaver digital micro-mirror device (DMD) has contacting elements that are reportedly not prone to stick together. In the case of a deflecting mirror device, landing electrodes are covered with a grating, which serves to reduce the contacting area but still permits conduction between the mirror and a landing electrode. Alternatively, the landing electrode can be fabricated as a grated surface.
2.7 Background Regarding Optomechanical Hinges, as May be Used in Optomechnical Switches
The optomechanical switch of the present invention will shortly be seen to preferably incorporate a micromechanical hinge of improved form. A general reference to the state of the art in these micromechanical structures is given in K. S. J. Pister, M. W, Judy, S. R. Burgett, and R. S. Fearing, xe2x80x9cMicrofabricated Hinges, Sensors and Actuators: A. Physical, Vol. 33, pp. 249-255, 1992.
Another reference is U.S. Pat. No. 5,600,383 to Hornbeck for a MULTI-LEVEL DEFORMABLE MIRROR DEVICE WITH TORSION HINGES PLACED IN A LAYER DIFFERENT FROM THE TORSION BEAM LAYER assigned to Texas Instruments Incorporated (Dallas, Tex.) The Hornbeck multi-level DMD (where here the initials xe2x80x9cDMDxe2x80x9d have their alternative meaning of xe2x80x9cdeformable mirror devicexe2x80x9d) concerns a bistable pixel architecture where torsion hinges are placed in a layer different from the torsion beam layer. This results in pixels which can be scaled to smaller dimensions while at the same time maintaining a large fractional active area, an important consideration for bright, high-density displays such as are used in high-definition television applications.
The present invention contemplates micromachined, or Micro Electro Mechanical Systems (MEMS), optomechanical switches of new design having certain advantages over existing MEMS optomechanical switches. These advantages include, inter alia, (i) increasing the speed, certainty and exactitude of (micro-) movement (which movement is, among other things, useful for the optical switching of a light beam), while (ii) reducing stiction, of sticking, of the components of the optomechanical switch.
The invention realizes these advantages through certain unique structures that are usable both in the preferred micromechanical optomechanical switches of the present invention and also in other MEMS switches and devices. These structures include (i) geometries of (micromachined) electrode plates and/or electrical coils which serve to develop electrostatic and/or electromagnetic forces so as to move a (micromechanical) switch element in (angular, pivoting) position, (ii) surface micromachined hinges which permit (micromachined) plate to be accurately angularly pivoted in a predetermined plane, (iii) (micromechanical) torsion springs that both mechanically bias the (micromachined micro-hinged) plate in its angular pivoting movement and that also serve to conduct electricity to the (micromachined angularly pivoting microhinged) plate, and (iv) a structure micromachined upon a substrate for both mechanically receiving, and for making electrical connection to, a distal end region of micromachined plate that is controllably pivoting about the substrate from its proximal end region so as to selectively land upon the structure, thus xe2x80x9ca micromachined landing electrode structurexe2x80x9d.
1. A micromachined Optomechnical Switch
1.1 Hinged for Pivoting Operation Under Electrical Force
In one of its aspects the present invention is embodied in a micromachined optomechanical switch having (i) a micromirror that is mounted to (ii) a micromachined plate (sometimes called a xe2x80x9ctorsion platexe2x80x9d) that is itself mechanically hinged about a pivot axis to (iii) a substrate. The switch is activated by a circuit selectively develops (i) an electrostatic or (ii) an electromagnetic force between the micromachined plate and the substrate, either of which (i) electrostatic or (ii) electromagnetic force causes the micromachined plate and the micromirror mounted thereto to pivot in angular position relative to the substrate.
By this pivoting operation a radiation beam will selectively intercept the micromirror dependent upon whether the micromachined plate is, or is not, moved in angular position by action of the electrostatic, or the electromagnetic, force that is selectively developed by the circuit between the micromachined plate and the substrate. Ergo, an electrically-controlled optomechanical switch is realized.
Note that the (i) micromirror is not the same as the (ii) pivoting micromachined plate. Indeed, the micromirror is preferably mounted (i) perpendicularly to the micromachined plate, and (ii) orthogonally to the pivot axis of the (pivoting) micromachined plate. By this orientation, and by this relationship, the micromirror will constantly move in the same plane during all angularly pivoting movement of the micromachined plate. This xe2x80x9csame-planexe2x80x9d operation is very useful both (i) to selectively direct an incident radiation beam that skims over the top of the substrate, (and that is typically at 45xc2x0 to the plane of the micromirror) in useful directions, and (ii) to maintain precision optical alignment.
1.2 Hinged for Pivoting Operation Under Electrostatic Force
In greater detail, one, preferred, embodiment of the micromachined optomechanical switch employ electrostatic force as its reliable, strong, and readily-realized actuation mechanism. It is possible to generate either (i) a repulsive electromagnetic force, or (ii) an attractive electromagnetic force, between the micromachined plate and the substrate by the simple expedient of placing an electrostatic charge on both the micromachined plate and the substrate that is, respectively, (i) of the same polarity, or (ii) of differing polarity.
In actual implementation, the generation of a repulsive electrostatic force requires placement of a same-polarity electrical charge on two conductors one or which is electrically floating, and this is more difficult to realize than is the placement of an opposite-polarity electrical charge on each conductor. Therefor the preferred MEMS devices of the present invention, as do the majority of electrostatic MEMS devices, mainly use an attractive electrostatic force, which attractive electrostatic force can readily be easily realized by applying a voltage between two conductors, in this case the (i) micromachined plate and (ii) the substrate.
In developing either the attractive or the repulsive type of electrostatic force, at least one of the micromachined plate and substrate is electrically conducting. Normally the micromachined plate is so conducting. The mechanism for selectively developing an electromagnetic force between the micromachined plate and the substrate requires selectively emplacing, and removing, an electrostatic charge. The electrostatic force developed in response to the electrostatic charge (or, more simply, an xe2x80x9celectrical voltagexe2x80x9d) operates so as to attract (alternatively, so as to repel) the micromachined plate from the substrate, causing it to pivot in angular position about a hinged pivot axis towards (alternatively, away from) the substrate.
An electrical insulator is positioned between (i) the substrate and (ii) the micromachined plate that is hinged to, and that pivots upon, the substrate. An electrical voltage selectively developed between, as a first electrode, (i) the substrate, and, as a second electrode, the (ii) micromachined plate, produces an electrostatic force that serves to pivot the micromachined plate and the micromirror mounted thereon relative to the substrate.
1.3 Hinged for Pivoting Operation Under Electromagnetic Force
Another, less preferred, embodiment of the micromachined optomechanical switch uses electromagnetic force as the actuation mechanism. In this case an electromagnet is positioned in one of (i) the substrate and (ii) the micromachined plate (that is hinged to, and that pivots upon, the substrate), while, typically, permanent magnet is placed in the remaining one of (i) the substrate and (ii) the micromachined plate. (It is possible to use an electromagnet in both (i) the substrate and (ii) the micromachined plate.)
Selectively energizing the electromagnet produces an electromagnetic force that serves, dependent upon the magnetic polarization, either to attract, or to repel, the permanent magnet, thus (in either case) pivoting the micromachined plate and the micromirror mounted thereon relative to the substrate.
In accordance with the principles of a common solenoid that a it is more effective to produce a repulsive electromagnetic force than an attractive electromagnetic force between an electromagnet and a permanent magnet that is appropriately aligned to the field of the electromagnet, in the preferred micromachined optical switches of the present invention the magnetic field of the permanent magnet is positioned and oriented relative to the electromagnet so that energization (at a predetermined direction of current flow) of the electromagnet serves to repel the permanent magnet. Note that, being repulsive in nature, the preferred electromagnetic force is opposite to the preferred electrostatic force, which is attractive in nature.
In fact, the numerous preferred embodiments of micromachined optomechanical switches in accordance with the present invention can be made so as to employ either an attractive or a repulsive force (or both, in certain switch embodiments) of either an electrostatic and/or an electromagnetic nature. Although certain force directions and force types are preferred for ease of implementation, a practitioner of the electrical arts will realize that all the various variations of construction are essentially equivalent.
Indeed, embodiments of the micromachined optomechanical switches in accordance with the present invention which use both attractive and repulsive forces at the same time, or at separate times, are eminently possible. If the attractive force is considered to be a xe2x80x9cpullxe2x80x9d, and the repulsive force to be a xe2x80x9cpushxe2x80x9d, then the term xe2x80x9cpush-pullxe2x80x9d is defined as xe2x80x9cpush OR pullxe2x80x9d, or those very xe2x80x9csingle electrical actionxe2x80x9d switches just discussed. However, the term xe2x80x9cpush and pullxe2x80x9d is reserved for switches that electrically both push AND pull. It is possible to construct a xe2x80x9cpush and pullxe2x80x9d switch having both attractive and repulsive forces (i) exerted in separate areas of the same switch at the same time, or (ii) exerted in the same or in separate areas of the same switch at separate times.
If both the xe2x80x9cpush and pullxe2x80x9d forces are exercised concurrently, then the switch will generally be very xe2x80x9cstrongxe2x80x9d and very xe2x80x9cfastxe2x80x9d to assume at least a desired forced position. Conversely, if the xe2x80x9cpush and pullxe2x80x9d forces are exerted in a time sequence, then the switch will clearly exhibit a positively-controlled action, assuming first one and then the other position each under a applied electrical force. Likewise, it is possible to build a hybrid xe2x80x9cpush and pullxe2x80x9d switch that combines, for example, electrostatic pull and electromagnetic push; again at the same time, or at separate times.
Continuing with the basic electromagnetic embodiment of the micromachined optomechanical switch, the electromagnet is preferably located in the substrate, thus making that the permanent magnet is located in the micromachined plate, for reason of ease of fabrication. However, the opposite positions are possible. The permanent magnet is preferably made from a magnetic material integrated within the micromachined plate, and is more preferably made of permalloy. The electromagnet is normally a simple loop of conductor, such as may be made from the traces upon a printed circuit substrate.
1.4 With the Pivoting Element Biased in Position to Reduce Stiction
Variants of both embodiments of the micromachined optomechanical switch permit (i) minimization of stiction, of the undesirable sticking of the micromechanical pivoting plate to the substrate, and (ii) maximization of the spatial movement of the mirror.
For example, in the magnetic embodiment just discussed, the permanent magnet in the micromachined plate can serve to bias the micromachined plate in position off the substrate. This bias will exist regardless that the electrostatic or the electromagnetic force that serves to repel (or to attract) the micromachined plate, and that causes it to pivot away from the substrate, may be absent. The positional biasing of the micromachined plate from off the substrate diminishes the possibility of stiction of the micromachined plate to the substrate.
This positional biasing is typically realized mechanically. In one embodiment of the micromachined optomechanical switch a three-dimensional surface-micromachined structure serves to physically support the micromachined plate in a position angled about the pivot axis and displaced off the substrate. Again, the biasing the pivoting micromachined plate in position off the substrate serves to diminish the possibility that the micromachined plate will fail in its pivoting motion by undesirably adhering to the substrate.
1.5 Push-pull, and Push and Pull, Switches
In another, embodiment, the micromachined plate is bent about an axis, and pivots about the substrate along this axis. As a consequence of this bend, only a portion of the micromachined plate that is upon a one side of the axis will (or can) be against the substrate at any one time, while the portion of the micromachined plate that is upon the other side of the axis will be angled away from the substrate.
The micromirror is mounted to only one bent arm portion of the micromachined plate. The bent micromachined plate serves like a rocker, alternately lifting and lowering the micromirror from the substrate.
Typically one (only) arm portion of the bent micromachined plate is biased in position by a torsion spring. The restoring force of this torsion spring is compounded by an electromagnetic or electrostatic force. This switch is called a xe2x80x9cpush-pullxe2x80x9d switch where it is remembered that this term means to electrically push (i.e., repulse) OR pull (i.e., attract). Clearly then the other push, or pull, force is provided by the spring.
However, it is. also possible to construct, as a variant of the pivoting micromachined plate xe2x80x9cpush-pullxe2x80x9d switch, a xe2x80x9cpush and pullxe2x80x9d switch. This xe2x80x9cpush and pullxe2x80x9d micromachined optomechanical switch permits that all movement of the rocker plate within the switch transpires under positive electrical control. This occurs because the electrical circuit for selectively developing a force selectively so develops a force that alternately repels each portion of the micromachined plate. These forces can be sequenced so as to positively force the micromachined plate to pivot in position: first so as to xe2x80x9cpush-the-first-portionxe2x80x9d followed by xe2x80x9cpush-the-second-portionxe2x80x9d. A rocking motion of the switch is thus induced in a xe2x80x9cpush and pushxe2x80x9d operation.
Alternatively, the forces between the substrate and each portion of the bent micromachined plate may be temporally sequenced so as to positively force the micromachined plate to pivot in position first in a xe2x80x9cpush-the-first-portion while pulling-the-second-portionxe2x80x9d operation, followed by a xe2x80x9cpull-the first-portion while pushing-the-second-portionxe2x80x9d operation, or a xe2x80x9cpush and pullxe2x80x9d operation. Clearly the xe2x80x9crockingxe2x80x9d bent micromachined plate, and the micromirror mounted thereto, are not only controlled in position, but may be made to strongly and quickly assume any desired position.
These variants are quite useful. An optical switching system designer will normally use a micromechanical xe2x80x9crocker switchxe2x80x9d that has the desired response time in each direction of its motion.
1.6 Even More Substantially Three-dimensional Pivoting Switches
Certain other three-dimensional structures are equally, or even more, efficacious for strong, fast, positive switch control.
In one three-dimensional embodiment a micromachined structure serves to elevate the micromachined plate above the substrate, and about a pivot axis that is intermediary in position within the micromachined plate between a first portion and a second portion thereof. (Normally but one micromirror is, as before, mounted to but one portion of the micromachined plate.)
The mechanism for selectively developing a force between the micromachined plate and the substrate so develops the force first between the substrate and one portion of the micromachined plate, and then between the substrate and the other portion. This causes the micromachined plate and the micromirror mounted thereto to pivot in angular position relative to the substrate (as before), only now about the elevated pivot axis.
In accordance with (i) the principle of a lever, and in accordance with the fact that (ii) the micromachined plate may be greatly pivoted in angle in its position elevated above the substrate, the micromirror may be greatly moved in position. In other words, small, or micro, machines can be made to reliably positively produce (relatively) large and useful motions! The (micro) force produced at the long end of the pivoting lever is, of course, minuscule. Luckily, it does not take any great force to reflect light in a mirror, nor to block the transmission of light by a opaque filter!
Normally when (micro) forces are minuscule, stiction can be a problem. Certain other structures within micromachined optomechanical switches in accordance with the present invention serve to minimize, or even eliminate, this problem.
2. A Micromachined Landing Electrode Structure
In accordance with another of its aspects, the present invention is embodied in a micromachined landing electrode structure. This is a structure micromachined upon a substrate for (i) mechanically receiving, and (ii) making electrical connection to, a distal end region of micromachined plate that is pivoting about the substrate at its proximal end region so as toxe2x80x94among such other things as were, for example, discussed abovexe2x80x94selectively land upon this structure (which is upon the substrate). The structure is thus called a xe2x80x9cmicromachined landing electrode structurexe2x80x9d.
This micromachined landing electrode structure is made from an electrically conductive buckled beam that is buckled above a plane of the substrate in a location that falls under the distal end region of the pivoting micromachined plate. The buckled beam serves (i) as a spring to the pivoting movement of the micromachined plate (at least when it draws near to the substrate), (ii) as an electrode for making electrical contact to the micromachined plate when the micromachined plate is pivoted into (pressured) contact therewith, and (iii) as a stop helping to prevent stiction between the pivoting micromachined plate and the substrate.
The electrically conductive buckled beam is itself preferably formed as polysilicon beam that is pushed until it buckles, and that is then locked in its buckled position into a micromachined structure.
The micromachined plate is normally biased in its angular position by a torsion spring and/or by an electrostatic or an electromagnetic force. When the micromachined plate is in full contact with the substrate, then the contact area is large and the stiction force between micromachined plate and the substrate could also be large. If this stiction force proved to be larger than the restoring force of a torsion spring serving to bias the pivoting micromachined plate in position, then the micromachined plate could become permanently undesirably stuck to the substrate. To combat stiction force, the contact area is reduced by employing the buckled beam, which reduces the contact area to basically a point. Additionally, the larger spring constant of the buckled beam can make the total restoring force larger than any residual sticking force, therefore further reducing the possibility of stiction.
The micromachined plate is normally biased in its angular position by a torsion spring. The spring constant of the buckled beam is made to be much larger than that of the torsion spring so that a force on the micromachined plate by the buckled beam (when the micromachined plate is in contact therewith) will be much stronger than an opposite force on the micromachined plate arising from the torsion spring. The anti-stiction restoring force may alternatively be realized by a cantilever bending spring that is cantilevered from the approximate center of the buckled beam so as to contact the micromachined plate when its pivots close to the substrate. Both bucked beam and/or cantilevered spring provide a spring force that serves to angularly kick the torsion plate off the substrate when any (electrostatic or electromagnetic) force holding it proximately to the substrate is released. This effect promotes fast mechanical action of the micromachined plate, and of the micromachined optomechanical switch of which the micromachined plate forms a part.
3. Improved Micromachined Hinges
In still yet another of its aspects, the present invention is embodied in hinges for a micromachined plate that is angularly hinged for pivoting upon and relative to a substrate.
The hinged pivoting micromachined plate mechanism includes (i) a micromachined plate having at least one substantially straight side adjacent to which straight side is relieved at least one hole. Only a narrow strip of the plate remains between this at least one hole and the edge of the straight side.
The mechanism further includes (ii) so many micromachined staples as there are holes are attached to the substrate. Each staple spans over the narrow strip of the micromachined plate and into a hole so that both ends of the staple can be permanently connected to the substrate. The mechanism thus formed holds the micromachined plate to angularly pivot about the substrate on its narrow strip, which serves as a hinge pin, and along its straight side.
Although these structural elements (i) and (ii) taken alone may well be different from previous micromachined structures, it should be recalled that, in accordance with the principles of the present invention, this pivoting, hinged, micromachined plate must also be both (i) supplied with electrical voltage (at least in the electrostatic embodiments of the switch) and (ii) mechanically biased in position (at least for those switch embodiments that are not xe2x80x9cpush and pullxe2x80x9d). The preferred hinged pivoting micromachined plate mechanism does so permit (i) the micromachined plate to angularly pivot on its straight side about the substrate while (ii) an electrical bias is separately provided to the micromachined plate.
This dual function is accomplished by the final structural element of (iii) one or more torsion springs each of which are attached between the substrate and a region along the straight side pivoting end of the micromachined plate. This (these) torsion spring(s) serves to mechanically bias the micromachined plate in angular position towards the substrate, and into but an acute angular separation therefrom. The angular displacement of the micromachined plate, as it pivots on its hinge pin, is normally limited by these one or more torsion springs.
The combination of (i) the straight-edged holed micromachined plate, (ii) the staples spanning such narrow portion of the micromachined plate as serves as the hinge pin, thus constraining the micromachined plate to pivot upon its straight edge, and (iii) the one or more torsion springs, combine to establish a well-defined axis and direction of angular rotation to the pivoting micromachined plate. In other words, the micromachined plate is exactingly positioned, and rotates exactinglyxe2x80x94properties which are useful for the precision deflection of light.
Moreover, the same (iii) one or more torsion springs also serve to conduct electricity between an appropriate electrical trace upon the substrate and, again, the same straight-side pivoting-end region of (i) the micromachined plate. Selective electrical energization proceeding along this conductive path is, or course, exactly how the hinged micromachined plate (and any micromirror attached thereto) is caused to variably selectively pivot in angular position relative to the substrate.
These and other aspects and attributes of the present invention will become increasingly clear upon reference to the following drawings and accompanying specification.